This is a known issue when setting null or undefined for the elements fill(fill: dataItem.Color, for new items) option. The problem is already fixed and the fix will be included in the next internal build. A workaround for now is to set a default value for the color when creating the visual:
fill: dataItem.Color ||
or for the Color field in the model:
m.Field(s => s.Color).DefaultValue(
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